Why does structural behavior change in different types of soil?

There’s a recipe for getting it right, depending on what you need…

By Sarah Jensen

As any 5-year-old can tell you, the secret to the perfect mud pie is the correct amount of water and the right kind of dirt. They know from experience that sand and clay behave differently and that using one or the other will spell the success or failure of a playhouse recipe.

The behavior and strength of soil is largely determined by the size of its particles and the migration of water through the soil skeleton, says Andrew Whittle, the Edmund K. Turner Professor who specializes in geotechnical engineering and geomaterials and is head of MIT’s Department of Civil and Environmental Engineering.

Soil types range from silt with particles in the sub-millimeter range, to clay, sand, gravel, and boulders, and most contain water, he explains. The larger the granules, the greater amount of water that can flow between them, determining the soil’s stability. “In essence, a change in water content drives change in soil strength,” says Whittle.

Water flows freely from sand, for example, in which the grains are rounded and pore spaces are large and numerous. As beachgoers can attest, their every step upon the shore squeezes water up through the sand. Water flows out, leaving the sand’s porous, unstable skeleton.

In clay, on the other hand, water moves much more slowly through its flat nanoparticles. “A lot of water is held on the surfaces of the tiny clay particles and isn’t free to move within its skeleton,” says Whittle. “The water holds the particles together through capillary force, providing additional strength.”

Understanding the behavior of various soils is of utmost concern to the civil engineer. “Soil strength is extremely important in the design of all support systems, from a building’s foundation to a highway overpass,” says Whittle, who served on the Massachusetts governor’s safety review panel of Boston’s Big Dig tunnels. “When we build highway embankments on clay, we rely on a principle known as consolidation of clay. We build one layer and allow the water to flow out so the clay gains strength before we add another layer.”

Farmers, too, must know how various soils behave if they expect to successfully reap what they sow. Natural soils are a blend of sand and clay, and their behavior depends on what’s in the mix. In fields made up of too much sand, rainwater washes nutrients through the porous soil skeleton and far from the spinach and sweet corn and soybeans. Too much clay and the water moves too slowly to transport necessary minerals the plants need to grow. While agricultural soils are not his specialty, Whittle says the principles remain are the same. “While particles in some soils are bonded together by minerals that re-precipitate in the soil, most rely on the interlocking and friction between particles, and capillary forces exerted by the pore water. It’s all about cohesion —the relative ability of water to move.”

It’s a principle mud pie makers know intuitively. Mix just the right proportions of water and clay and allow it to dry a bit, and the pie will stand up very well on the plate. But stir in too much sand, and it crumbles like a sandcastle on the sunny seashore.

Thanks to Mahabubar Rahman from Dhaka, Bangladesh, for submitting this question.